1. Field of the Invention
The present invention relates to a video/audio decoding technique in conformity with ATSC (Advanced Television Systems Committee) standard, and more particularly to an equalization in a signal receiving system.
2. Description of Related Art
As a modulation system for a terrestrial digital broadcasting in the United States, the Trellis-encoded 8VSB modulation in conformity with the ATSC standard is employed. The received signal of the terrestrial broadcasting includes a lot of echo signals, and the configuration of the adaptive equalizer greatly affects the demodulation performance. As a result of field tests carried out until now, it has been pointed out that the high-speed adaptive equalization by using a Decision Feedback Equalizer (DFE) having about 256 taps is required.
In the configuration of DFE, the estimation accuracy of the estimated transmission symbol value, which is used for echo-signal cancellation and the adaptive updating of the equalization coefficients, is important. In a conventional equalizer configuration, a simple 8-value slicer has been used for the purpose of the transmission symbol estimation. However, in such a case, the estimation accuracy of the transmission symbol is low, and appropriate equalization cannot be achieved around the allowable error rate limit of the system.
It is an object of the present invention to provide a decision feedback equalizer for use in a signal receiving system, which is capable of performing high-performance equalization compared with the conventional DFE with 8-value slicer, without an unacceptable increase of the hardware scale.
According to one aspect of the present invention, there is provided a decision feedback equalizer for use in a digital signal receiving system, including: an adder for subtracting an estimated echo from an input symbol to generate a demodulated symbol; a Trellis decoder for applying Trellis decoding onto the demodulated symbol to generate maximum likelihood transmission symbol sequence; a feedback register for storing estimated transmission symbol values for a feedback filter; a coefficient register for storing coefficient values for the feedback filter; a feedback register updating unit for generating an estimated transmission symbol and the estimated transmission symbol value corresponding to a next time point based on the demodulated symbol, the maximum likelihood transmission symbol sequence and the estimated transmission symbol corresponding to a current time point, and for updating the values stored in the feedback register by using the estimated transmission symbol value corresponding to the next time point; a coefficient updating unit for updating the values stored in the coefficient register based on the demodulated symbol and the estimated transmission symbol; and an estimated echo calculator for calculating the estimated echo based on the values stored in the feedback register and the values stored in the coefficient register.
In accordance with the equalizer thus configured, the adder subtracts the estimated echo from the input symbol to generate the demodulated symbol, and the Trellis decoder applies Trellis decoding onto the demodulated symbol to generate maximum likelihood transmission symbol sequence. The feedback register stores estimated transmission symbol values for a feedback filter, and the coefficient register stores coefficient values for the feedback filter. The feedback register updating unit generates the estimated transmission symbol and the estimated transmission symbol value corresponding to the next time point based on the demodulated symbol, the maximum likelihood transmission symbol sequence and the estimated transmission symbol corresponding to a current time point, and updates the values stored in the feedback register by using the estimated transmission symbol value corresponding to the next time point. The coefficient updating unit updates the values stored in the coefficient register based on the demodulated symbol and the estimated transmission symbol. The estimated echo calculator calculates the estimated echo based on the values stored in the feedback register and the values stored in the coefficient register.
Thus, the values stored in the feedback register are updated by the highly-accurate maximum likelihood transmission symbol sequence which is generated by Trellis-decoding the demodulated symbol, and hence the optimum echo cancellation and equalization can be achieved.
The Trellis decoder may output a decoding result corresponding to the maximum likelihood transmission symbol sequence. Thus, the decoding result may be obtained without providing a separate Trellis decoder at the downstream of the equalizer. This enables downsizing of the receiver.
The feedback register updating unit may update the values stored in the feedback register by using the maximum likelihood transmission symbol sequence. Thus, the values stored in the feedback register are updated at every moment based on the maximum likelihood transmission symbol sequence, and hence more accurate estimated echo valued may be obtained.
The feedback register updating unit may calculate the estimated transmission symbol corresponding to a past input symbol by using the maximum likelihood transmission symbol sequence which is calculated by the Trellis decoder and corresponds to input symbol until a current time point, and may update the values stored in the feedback register by using the estimated transmission symbol corresponding to the past input symbol. By this, the feedback register updating unit generates the highly-accurate estimated transmission symbols corresponding to the past symbols by using the newly obtained maximum likelihood transmission symbol sequence, thereby improving the echo canceling accuracy.
In a preferred embodiment, the Trellis decoder may apply the Trellis decoding only to data symbols included in the demodulated symbols. In another preferred embodiment, the Trellis decoder may receive a frame counter value indicating a position of the demodulated symbol within a data frame, and may apply the Trellis decoding only to the data symbols based on the frame counter value. In still another preferred embodiment, the feedback register updating unit may receive a frame counter value indicating a position of the demodulated symbol within a data frame, and may update the values stored in the feedback register by using the estimated transmission symbol corresponding to the past input symbol, only for the data symbols based on the frame counter value. Thus, it is ensured that the Trellis-decoding is applied only to the data symbols.
The maximum likelihood transmission symbol sequence may be an aggregation of the maximum likelihood estimated transmission symbols corresponding to current and past input symbols. Thus, the values stored in the feedback register may be stored in consideration of the estimated transmission symbol corresponding to the past input symbol.
The coefficient updating unit may update the values stored in the coefficient register corresponding to a next time point based on the demodulated symbol, the estimated transmission symbol, the estimated transmission symbol value of a current time point and the coefficient value of the current time point. Also, the data symbol corresponding to the input symbol supplied to the equalizer may be interleaved by a plurality of Trellis encoders, and the Trellis decoder may include a plurality of Trellis decoding units for de-interleaving the demodulated symbol.